This invention relates to apparatus and methods for controlling and maintaining a stable difference in electrical potential between two selected elements of an electrochemical device. Electrical circuits for achieving such a controllable and stable potential are called potentiostats, and are most commonly used with amperometric devices for measuring the concentrations of specific substances in the gas or liquid phase. A second function of the potentiostat is to provide an output signal that is representative of the quantity of the substance being analyzed. The amperometric device and the potentiostat together form a chemical measuring system; both components are essential to the measuring operation.
Amperometric gas sensors often use sensing, or working, electrodes made of finely divided catalyst material, which has a large surface area per unit weight. The electrical capacitances of these electrodes are very large. With conventional potentiostat circuits, the sensor-potentiostat system often becomes unstable and oscillates. It is the object of this invention to provide a potentiostat circuit whose stability is not affected by large capacitances in the sensor.
An amperometric sensor consists of either two or three electrodes immersed in an ionically conducting medium, which is usually an electrolyte solution. In the three-electrode version, the electrodes are named the working electrode, the reference electrode, and the auxiliary electrode. The working electrode is the site of the analytical reaction; in a gas sensor, it is exposed to the sample of the gas being monitored. The reference electrode is designed to be at a constant electrical potential relative to the electrolyte solution. In the three-electrode version, it is important to the functioning of the reference electrode that no current flows in it. In such cases,the auxiliary electrode is used to provide an electrical current to the electrolyte solution that is equal and opposite in sign to the current at the working electrode.
In a two-electrode sensor, a current may flow through the reference electrode, and the functions of the reference electrode and auxiliary electrode are combined in one electrode. This is called the counter electrode.
A potentiostat circuit always contains a feedback loop, in which the potential of the reference (or counter) electrode is measured relative to the working electrode. This measured potential (cell potential) is combined with a desired potential (the set-point potential) to obtain an error signal in the form of a voltage. In the potentiostat circuits that are conventionally used with amperometric sensors, the error voltage is applied directly to the auxiliary (or counter) electrode, causing a current to flow. When the system is at equilibrium, the reference potential relative to the working electrode is equal in magnitude to the set-point potential.
Such circuits provide a way of controlling the operation of the sensor, but do not provide a way of obtaining an output signal. To obtain an output signal, the feedback loop must be compromised in one of two ways. In one way, the working electrode is connected to a current-to-voltage converter circuit. At equilibrium, this circuit forces the working electrode to maintain a potential near ground potential. The reference (or counter) electrode voltage is therefore measured relative to ground. Because the response time of existing operational amplifiers is not infinitely fast, the voltage of the working electrode will deviate from ground when the current flow changes at a rapid rate. This results in an erroneous measurement of the cell potential which causes a fluctuation in the voltage applied to the counter electrode. The result may be a high-frequency oscillation, which is rendered more likely by large electrode capacitances. In addition, noise may be increased if a long cable is connected between the sensor and the potentiostat.
The second way of obtaining an output signal is to insert a current-sensing resistor in series with the auxiliary or counter electrode. The voltage across the sensing resistor is directly proportional to the magnitude of the current flowing through the counter electrode. A differential amplifier is connected across the sensing resistor and the output of the differential amplifier becomes the output signal. The problem with this method is that a large value of resistance must be used to provide a measurable signal. A large sensing resistor could limit the voltage available to drive the counter electrode for maintaining a constant potential relative to the electrolyte solution. If a smaller sensing resistor is used, the gain of the differential amplifier must be increased to accommodate the smaller voltage across the sensing resistor. Additional gain in the differential amplifier can introduce error signals due to common mode rejection, impedance loading of the sensor, and increased noise signal or else increase significantly in the number of operational amplifiers needed for the electrical circuit.
It is object of this invention to provide a potentiostat circuit in which the working electrode of the sensor is connected directly to ground, so as to improve the sensor's signal-to-noise ratio.
It is another objective of this invention to provide a potentiostat circuit for an amperometric sensor with a directly grounded working electrode that permits measurement of very small currents typically generated in amperometric gas sensors without resorting to a high-gain differential amplifier.
It is still another object of this invention to provide a potentiostat circuit that will operate in a stable manner with an amperometric sensor whose working electrode has a high capacitance.
It is yet another object of this invention to provide an amperometric sensor with a potentiostat circuit that allows the operating parameters of the feedback loop to be controlled apart of the operating parameters of the circuit that generates the output signal. For example, the frequency response of the feedback loop of the invention may be deliberately controlled without affecting the design of the circuit for generating the output signal.